Since a ZnO crystal is a direct transition semiconductor whose bandgap is about 3.37 eV, its bound energy of the exciton which electrons and holes combined within the solid is as large as 60 meV, and it exists stably also at room temperature, the ZnO crystal is an affordable price, and its environmental load is also small. Therefore, the ZnO crystal is expected as a light-emitting device from a blue light wavelength region to an ultraviolet light wavelength region.
The ZnO crystal of a use is wide except a light-emitting device, and therefore the application to a light-detecting element, a piezoelectric element, a transistor, a transparent electrode, etc. is also expected. In order to use the ZnO crystal for the above-mentioned uses, the establishment of ZnO crystal growth technology of high quality excellent in mass production nature is very important, and the doping technology for controlling the conductivity of a semiconductor is also important.
The following methods are known as a method of fabricating a ZnO based semiconductor of high quality. For example, in the MBE (Molecular Beam Epitaxy) method, the ZnO based semiconductor of high quality is grown up by supplying molecular beam of the zinc and the oxygen radical (plasma), and reacting the supplied zinc and oxygen on a growth substrate. Moreover, in the PLD (Pulsed Laser Deposition) method, the ZnO based semiconductor of high quality is grown up by illuminating the sintered body and crystal of the ZnO based semiconductor with a laser beam, and depositing an evaporated ZnO based semiconductor on a growth substrate.
However, since a large area film formation is difficult and needs to grow in a vacuum when growing up the ZnO based semiconductor by the MBE technique and the PLD method mentioned above, there is a problem that it is difficult to mass-produce industrially.
Then, the method of growing up the ZnO based semiconductor by the MOCVD (Metal Organic Chemical Vapor Deposition) method widely used for the crystal growth of a group III-V semiconductor is known as a fabrication method of the ZnO based semiconductor which does not need a high vacuum. In the MOCVD method, an organic metal including the zinc is decomposed near the substrate or on the substrate, finally the oxygen material reacts to the metallic elements, and thereby the ZnO based semiconductor is grown up.
However, in the MOCVD method mentioned above, since the vapor pressure of the zinc which is a group II element is extremely high compared with a group III element, even if the zinc reaches the growth substrate, it is easy to separate from the growth substrate, under the high temperature in which growth of high quality is possible. Accordingly, since the rate of the zinc which can be contributed to the growth of the ZnO based semiconductor on the growth substrate is small, there is a problem that the efficiency of material including the zinc is low.
Moreover, since carbon mixes into the ZnO based semiconductor by the hydrocarbon group generated when the organic metal material including zinc is resolved, there is a problem that the growth of the ZnO based semiconductor which does not include carbon is difficult.
When growing up the ZnO based semiconductor by the VPE (Vapor Phase Epitaxy) method, using a zinc single metal substance and oxygen material (for example, oxygen) including oxygen as a material is known as one method. However, since the equilibrium constant of the chemical reaction is large compared with the equilibrium constant of the group III-V semiconductor, and it is necessary to set up highly supplied partial pressure of the zinc with high vapor pressure for high temperature growth as mentioned above, there is a problem that it is difficult to control the reaction.
Then, the method using a zinc chloride and oxygen material as the alternative method in the case of growing up the ZnO based semiconductor by the VPE method is disclosed (for example, refer to Non Patent Literature 1). In the fabrication method of the ZnO based semiconductor according to the Non Patent Literature 1, the ZnO based semiconductor is grown up by installing the powder of zinc chloride in a reaction tube, carrying the zinc chloride which became steam by annealing by using carrier gas, and reacting the zinc chloride to oxygen.
The above-mentioned method is called an HVPE (Halide/Hydride Vapor Phase Epitaxy) method using halogenated group II metal as a group II material. In addition, the HVPE method is known as a fabrication method for the group III-V semiconductor which fabricates a gallium nitride substrate etc. industrially by using a halogenide (chloride) for group III material, and using a hydride for group V material. In this HVPE method, the hot wall method, which makes high temperature not only the growth substrate and its circumference but also a quartz tube, is generally used.
In the above mentioned Non Patent Literature 1, however, the fabrication method of the ZnO based semiconductor uses zinc chloride as zinc material and the zinc chloride is deliquescent. Furthermore, since the purity of the zinc chloride which can be obtained easily is as low as about 99.9% and the zinc chloride with high purity is expensive, the ZnO based semiconductor of high quality cannot be fabricated easily.
When using the ZnO substrate which is homogeneous material species as a substrate for ZnO crystal growth, the substrate for the ZnO crystal is produced with the hydrothermal synthesis method of the same method as the production of quartz crystal. There is a problem that the impurity control of the high level required in the semiconductor field is needed etc., and development of the hetero growth technology for growing up on a heterogeneous substrate is also needed.
When the nitride based semiconductor is used for the use of a LED (Light Emitting Diode) of which cost reduction is required in particular, a sapphire substrate, a silicon carbide substrate, a silicon substrate, etc. are used instead of using the expensive substrate for homo epitaxial crystal growth.
When the high temperature growth is directly performed on the sapphire substrate by the MBE method or the MOCVD method, there is a phenomenon in which the ZnO does not grow, and it is a phenomenon not occurring in GaN.
It is because it is difficult to grow up the ZnO at high temperature as for the element with low wettability on the substrate, such as Zn, if the wettability is not yet improved by forming the ZnO film on the substrate at low temperature firstly. On the other hand, since the group III element represented by Ga and Al has low vapor pressure and the wettability on sapphire is effective in the high temperature region, the crystal grows at high temperature without via a buffer layer.
Since diffusion of the materials on the surface of the substrate becomes difficult to occur and the growth film became an assembly of the rod-shaped or core-shaped crystal when growing up at low temperature, the high quality crystal growth of the semiconductor level was difficult although it is an orienting film.
Therefore, a method of growing up via a buffer layer is usually used for the epitaxial growth on the heterogeneous substrate. In particular, in the case of the GaN on the sapphire substrate, it has succeeded in obtaining the GaN crystal in which the crystallinity is effective and the surface flatness is excellent by forming the buffer layer comprising AlN or GaN at low temperature, and then growing up at high temperature (for example, refer to Patent Literature 1 and Non Patent Literature 2).
The technology of forming the buffer layer is proposed in the case of the ZnO based semiconductor crystal as well as the case of the GaN based semiconductor crystal (for example, refer to Patent Literature 2 and Non Patent Literature 3). According to the above-mentioned method, after growing up the low-temperature grown ZnO single crystal layer of about 10 nm to about 100 nm in thickness at the temperature lower than 600 degrees C. and then performing planarization processing by annealing, the ZnO growth is performed at the temperature lower than 800 degrees C. However, in the above-mentioned method, it is premised on the MBE technique and the growth temperature of high temperature growth is limited at about 800 degrees C. Moreover, according to a method of using a Zn single material or an organic metal material as a Zn material source, there was a problem that raw material efficiency decreased sharply in accordance that the high temperature growth is performed, because of the high vapor pressure of Zn.
When laminating the ZnO semiconductor film for a light emitting device and a light-detecting device unlike a transparent conductive film, a film of high quality with fewer defects is required. In the case of the GaN based semiconductor layer, although the device produced by the hetero epitaxial growth on the sapphire substrate is mass-produced, the dislocation density of its film is as much as not less than 108 cm−2 grade, and it is a level of dislocation density which cannot expect the usual device operation in any semiconductor layers except the GaN based, in particular an InGaN film.
The ZnO substrate produced by the hydrothermal synthesis method and the chemical vapor transport method are available, and in order to grow up the ZnO based semiconductor layer with fewer crystal defects, it is preferable to homo epitaxially grow by using the ZnO substrate. Since not only the lattice constant of growth film and the ZnO substrate are matched, but also the coefficient of thermal expansion of the ZnO substrate and the growth film is the same, the homo epitaxial crystal grown method is a method very excellent as film formation of the semiconductor layer with fewer crystal defects.
In recent years, although the ZnO substrate produced with the hydrothermal synthesis method has a problem in respect of an impurity control, the crystallinity measured from X-ray diffraction is high enough also as a semiconductor use, and since the ZnO film which grows homo epitaxial crystal layer performing lattice matching on the ZnO substrate may inherit the satisfactory crystallinity of the substrate, the growth of the ZnO based semiconductor layer with high internal quantum efficiency is expected.
The homo epitaxial growth on the ZnO substrate is reported with the PLD method, the MBE method, the MOCVD method, etc. The vapor phase epitaxial crystal growth such as the MOCVD method also among the above-mentioned methods is more fit for the mass production since an ultra-high vacuum is unnecessary, and the crystal growth is controllable by the gas supply volume which is easy to control. Therefore, the establishment of a vapor growth method in which the satisfactory homo epitaxial growth suitable for the mass production also with the ZnO based semiconductor is possible, and the development of a vapor phase epitaxial crystal growth apparatus are desired.
Although there are a lot of reports of the ZnO growth using the MOCVD method, there are few reports of the high temperature growth which can expect the crystalline improvement. This is because the organic metal decomposes near of the substrate into the single zinc substance, and the growth becomes difficult because of low stickiness coefficient of zinc, in the MOCVD method of the high temperature region (>800 degrees C.) of ZnO.
Moreover, the reactivity of organic metal, such as DMZn (dimethyl zinc) and DEZn (diethyl zinc), and oxygen material is high, and in growth by the pressure of about several 100 Torr adopted by usual MOCVD for group III-V semiconductor, the oxygen material reacts to the organic metal easily in the vapor phase before gas reaches a substrate (premature reaction). As a result, it becomes a cause of a jam of a source outlet unit and particle of a materials outlet unit.
There is a report that the high temperature growth is performed using the MOCVD method (for example, refer to Non Patent Literature 4).
In Non Patent Literature 4, it is growing homo epitaxially on the ZnO substrate using DMZn and O2 gas. There is a report that the surface where an atomic step appears by combining a high VI/II ratio and the substrate temperature of 1000 degrees C. is obtained. According to the result of an image of the AFM (Atomic Force Microscope) in Non Patent Literature 4, the direction of step is not match such as the ZnO substrate. Furthermore, a result that the height of step is also higher compared with a monolayer step is obtained. Moreover, an abnormality part of hexagonal prism shape with the size of about 50 nm appears, and further improvement is required for the homo epitaxial growth of high quality, as described in Non Patent Literature 2.
Moreover, according to the present inventors' experience, in the MOCVD method, the growth rate reduces as the high temperature growth is performed, and if the material partial pressure is increased in order to avoid the reduction of the growth rate, the material utilization efficiency is reduced by the above-mentioned premature reactions. As a result, there is a problem that realistic material utilization efficiency is not obtained.
In particular, the matter for which the ZnO is applied as p type becomes a great barrier of ZnO device development, and many organizations are concentrating on applying ZnO as p type even currently. As for the p type doping materials to the ZnO based semiconductor, a method of displacing an oxygen atom to a group V element is examined by many organizations, and N (nitrogen), As (arsenic), P (phosphorus), Sb (antimony), etc. are mentioned to a candidate. Also among this candidate, the ion radius of N is the same extent as oxygen, and N is leading as the p type dopant candidate of ZnO.
It is known that many acceptors in a wide gap semiconductor are trapped in deep levels usually, and an activation rate is low near room temperature. For example, the case of gallium nitride having a bandgap of the same extent as ZnO, the activation rate in the room temperature of Mg which is a p type dopant is as low as several percent, and in order to achieve carrier concentration (the concentration exceeding 1×1017 cm−3 is needed) used with optical devices, Mg more than 1019 cm−3 grade is doped usually.
It has been considered conventional that high doping of the nitrogen to ZnO is difficult in a high temperature region (for example, refer to Patent Literature 3).
For example, in Patent Literature 3, it is proposed the method of: forming a ZnO layer doped with high-concentration N at the low temperature which is about 300 degrees C. which nitrogen can dope mostly; and repeating the sequence which anneals at high temperature of about 800 degrees C. and forms a low concentration N doping layer. In Patent Literature 3, although the PLD method for annealing the substrate by laser is adopted, and it is a method in which rapid temperature increasing and rapid temperature decreasing are possible in a short time of several minutes, the step for growing ZnO is included also in the temperature increasing and temperature decreasing of a sample, and in particular, therefore it is difficult to control temperature with sufficient reproducibility during the temperature decreasing which is self-cooling.
As mentioned above, although the doping efficiency of nitrogen is strongly dependent on the growth temperature, since the crystallinity reduces and nitrogen is not activated if the substrate temperature is reduced, it is very difficult to form the p type ZnO.
Moreover, although there is also a MBE apparatus in the high vacuum process apparatus for performing nitrogen doping besides the above-mentioned PLD apparatus, N plasma using a radical cell for N doping source is used in many cases. Thus, in the apparatus using the radical cell, if plasma power is raised in order that a radical element is increased, there is a fault that sputtering of the internal wall of the cell is performed and an internal wall material is doped in ZnO.
If the internal wall material is doped in ZnO, it will also become a pollution source in many cases. As a result, there was a problem that it is not only difficult to obtain desired composition and p type doping, but also the controllability of ion concentration is difficult by introducing an impurity which is not aimed.
On the other hand, in the MOCVD method, there is a problem that the rate of the zinc which can be contributed to growth of the ZnO based semiconductor on the growth substrate is small, and the efficiency of material including zinc is low, according to the problem of zinc vapor pressure.
Moreover, since carbon mixes into the ZnO based semiconductor by the hydrocarbon group occurred when organic metal material gas including zinc is decomposed, there is a problem that growth of the ZnO based semiconductor which does not include carbon is difficult.